1. Field of the Invention
The present invention relates to electric lamps having an elongated tubular portion for inserting directly into a lamp mount or reflector base. More particularly, the present invention relates to electric lamps having an elongated tubular portion of a precise, predetermined length with respect to the optical center of said lamp, wherein at least a portion of said tubular portion is inserted directly into and secured in a bore of predetermined length in a plastic lamp mount or reflector so that the optical center of the lamp is at the focal point of the reflector without the need for adjustment, and mount and reflector assemblies containing such lamps.
2. Background of the Disclosure
There is much interest in the automobile industry in using tungsten-halogen lamps and arc lamps as the light sources for automotive headlamps. Tungsten-halogen lamps are presently in such use. Arc lamps have potentially longer life and higher light output and, further, the size of such arc lamps, such as metal halide arc discharge lamps, required for such lighting applications is relatively small, thereby enabling automotive manufacturers a greater leeway in innovative automotive design.
Tungsten-halogen lamps presently employed for automotive lighting in standard sealed beam headlamp units are generally welded to formed wires or posts which are then soldered or brazed to the lamp reflector through electrical feed-through members. Federal regulations are very stringent with regard to strength requirements for lamp sources for replaceable or composite lamps. Accordingly, such lamps are usually retained to a fixture by means of a strap member which is then welded to a metal member for the purpose of focusing and retaining the lamp in the base and in the reflector. U.S. Pat. No. 4,470,104 discloses a means for mounting a tungsten-halogen lamp wherein the lamp, due to temperature and other considerations, is held in place by metal members. Still another means for mounting a tungsten halogen lamp in an automotive type lamp assembly is disclosed in U.S. Pat. No. 4,754,373 in which the lamp is held in place by metal members proximate to the lamp.
In replaceable headlamps the position of the lamp filament relative to the lamp mount inserted into the rear of the reflector must occur within very narrow limits in order to have the focal point of the filament positioned reasonably close to the focal point of the reflector after the lamp and mount assembly are attached to the reflector. To obtain this kind of precision using existing technology, a complicated mounting arrangement is required so that the lamp may be moved relative to the lamp mount or base in order to position the optical center of the lamp within specified limits relative to the mount structure and then welded or otherwise secured to the mount. The lamp and mount assembly is then attached to the reflector in a precise fashion so that the optical center of the lamp corresponds with the focal point or optical center of the reflector. Examples of such complicated lamp mount structures and their use with reflectors are disclosed, for example, in U.S. Pat. Nos. 4,774,645; 4,795,388 and 4,795,936.
In contrast to tungsten-halogen lamps, arc discharge lamps, such as metal halide arc discharge lamps, require extremely high starting voltages, usually in the range of 10,000 to 20,000 volts. Because of these high voltages, it is necessary to electrically isolate the lead wires which exit the quartz or glass lamp envelope. Additionally, some of these lamp designs require very high starting frequencies in the order of 50 kHz in order to initiate the arc and at these high frequencies metallic parts in the proximity of the hot lead wire tend to increase the capacitance of the system. The result of this increased capacitance is to decrease the level of voltage delivered to the lamp for the purpose of initiating the arc. Further, corona discharge sometimes occurs between the hot lead wire and metal parts proximate to the lamp or lead wire. Accordingly, it is therefore desirable to limit the capacitance of the system by removing all but absolutely essential metallic elements from around the lamp. The use of metallic straps around the arc tube seal or otherwise in the proximity of the arc or high voltage lead would reduce the ability of the lamp to start or require higher voltages and, thus, more expensive electronics for starting a lamp in order to compensate for capacitance losses.
Still another phenomena which complicates the use of a scheme for supporting a metal arc discharge lamp relates to sodium loss from the arc chamber. Most arc tubes require compounds of sodium and one or more halogens to enhance their efficiency. Under certain conditions sodium ions can migrate through the quartz (or high temperature glass) arc chamber walls and the corresponding loss of sodium in the lamp results not only in hard starting or failure to start but darkening of the lamp envelope. Sodium migration out of the arc chamber also seems to be enhanced by the presence of metals near the arc chamber. This is a well known phenomenon in the lamp industry and larger metal halide lamps are designed to avoid or minimize the presence of metal near the arc chamber.